Process for benzophenone tetracarboxylic acid

ABSTRACT

A method is described for autoxidizing particular ortho-dialkyl substituted aromatic compounds using a particular type of solvent which gives rise to the formation of 3-alkanoyloxyphthalide compounds, some of which are novel. 
     The phthalides in turn are ionically oxidized to their corresponding aromatic polycarboxylic acids. 
     Combination of the two methods provides a means for converting ortho dialkyl substituted aromatic compounds directly to the corresponding polycarboxylic acids in higher yields and at generally lower overall temperatures and reaction conditions compared to prior art methods. 
     Polycarboxylic acids so obtained are known to be useful in the preparation of alkyds, polyesters, and the like, and, particularly, in the formation of the corresponding acid anhydrides which are used in the preparation of organic high temperature polymers such as polyamides, polyamideimides, and polyimides.

This application is a division of application Ser. No. 460,177 filedJan. 24, 1983, now U.S. Pat. No. 4,485,247.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to novel oxidation methods and is moreparticularly concerned with the ionic oxidation of certain phthalidemoieties to aromatic polycarboxylic acids, a method for the autoxidationof certain aromatic compounds having ortho dialkyl substituents to saidphthalides, particular novel diphthalides produced therefrom, and animproved method for converting said aromatic compounds to saidpolycarboxylic acids by the combination of the above two oxidationmethods.

2. Description of the Prior Art

Autoxidation processes involving the conversion of a wide variety ofpolyalkyl-substituted aromatic compounds to the corresponding aromaticcarboxylic acids have been extensively studied and documented in theprior art. Generally speaking, all of these methods produce directly thefully oxidized carboxylic acids.

Bruson et al in U.S. Pat. No. 2,806,059 disclose that diaryl methanes ordiaryl ketones in which the aryl rings are substituted by alkyl groupsand combinations of alkyl groups with carbonyl containing groups (i.e.aldehyde, acid, ester) can be oxidized in acetic acid solution withoxygen using, typically, cobalt salts as catalysts and aliphatic ketonesas promoters, to the corresponding carboxylic acids.

Saffer et al in U.S. Pat. No. 2,833,816 disclose the preparation ofaromatic polycarboxylic acids by oxidizing polyalkyl aromatic compoundswith oxygen in solution (preferably acetic acid) using a heavy metaloxidation catalyst in combination with bromine either in elemental,combined or ionic form.

In U.S. Pat. No. 3,038,940, Serres et al disclose the oxidation ofdiaryl substituted methylene groups to the corresponding diaryl ketonesin a solution of an oxidation resistant monocarboxylic acid in thepresence of the combination of a heavy metal oxidation catalyst andbromine.

In U.S. Pat. No. 3,089,906 Saffer et al disclose a process carried outabove atmospheric pressure wherein alkyl-substituted aromatic compoundsare oxidized in solution in the presence of a heavy metal oxidationcatalyst and a source of bromine.

Broadhead in U.S. Pat. No. 3,652,598 discloses the oxidation of various2,2',3,3'- and 3,3',4,4'-tetraalkyldiphenylmethanes to the corresponding2,2',3,3'- and 3,3',4,4'-tetracarboxylic acids by oxidation of thesubstrate with oxygen in acetic acid solution using, inter alia,manganese bromide as catalyst as taught in U.S. Pat. No. 2,833,816 citedsupra.

Jones et al (U.S. Pat. No. 3,162,683) in reporting on the oxidation ofalkyl aromatic compounds in the presence of perhalogenated aliphaticcarboxylic acids noted, in the case of o-xylene, that o-toluic acid wasformed along with varying proportions of phthalide. However, neither thecomplete oxidation of xylene to phthalide, nor the oxidation of bothmethyls of the xylene to carboxyl groups was found.

In the typical prior art cited supra, the methods for oxidizing thepolyalkyl-substituted aromatic compounds provide satisfactory yields ofthe aromatic acids when the alkyl groups are not on adjacent carbonatoms of the same aromatic ring. However, where the alkyl groups are inortho relationship to each other (i.e. on adjacent carbon atoms of thearomatic ring) the overall yields of the fully oxidized productsproduced by these prior art methods are low.

Apparently, the oxidation of the first alkyl group to a carboxylic grouphas the effect of deactivating the adjacent alkyl group thereby slowingdown, or stopping completely, the oxidation of the second alkyl.Generally speaking, product mixtures are obtained which contain mono-,di-, tri-, or tetracids depending on the starting number of alkyl groupsand the extent of oxidation. For example, when oxidizing the 2,2',3,3'-or 3,3',4,4'-tetraalkyldiphenylmethanes set forth in the processdescribed in U.S. Pat. No. 3,652,598 cited supra the corresponding puretetracids are not obtained but rather mixtures comprised of the mono-,di-, tri-, and tetracids. Consequently, yields of the desired tetracidare lowered and purification steps become complicated.

These disadvantages have been partially overcome in the prior art byresorting to much more rigorous oxidation conditions in terms of thereagents employed as typically disclosed in U.S. Pat. Nos. 3,078,279 and4,173,573 which call for the use of nitric acid at elevated temperaturesand pressures (i.e. 110° C. to 350° C. and up to 500 pounds per squareinch). While yields of desired tetracids are superior to those from theother methods discussed above, the more stringent operating conditionsrequired because of the nitric acid under the reaction conditions ofhigh temperature and pressure make for an expensive and somewhatdangerous procedure.

Surprisingly, it has now been discovered that a certain class ofphthalide compounds can be oxidized to the corresponding aromaticcarboxylic acids under mild ionic oxidizing conditions of aqueousalkaline hypohalite. The yields and product purity in regard to fullyoxidized products are superior to the direct autoxidations of the priorart discussed above yet the conditions are far less stringent than thoseprior art methods which employ nitric acid.

The only reference of which I am aware concerning a related method isU.S. Pat. No. 4,323,700 wherein a different class of phthalides areoxidized under ionic conditions to products unrelated to the instantorthodicarboxylic acids.

Further, it has also been discovered that, when a certain class ofaromatic compounds having dialkyl groups substituted on adjacent carbonatoms of an aromatic ring are subjected to autoxidation techniquessimilar to those described above in the prior art but differing in onekey respect, the result is not the direct formation of the correspondingpolycarboxylic acid but rather of an alkanoyloxyphthalide. The keydifference is the carrying out of the autoxidation in a solution of analiphatic carboxylic acid anhydride. The anhydride plays more of a rolethan just a solvent and this role will be discussed in detail below.

Furthermore, the above steps can be combined to provide an improvedmethod for oxidizing the aromatic compounds referred to above to thecorresponding aromatic polycarboxylic acids in yields and product puritywhich exceed the prior art direct autoxidation methods while at the sametime avoiding the use of nitric acid.

In a further unexpected advantage to flow from the combination of thetwo methods in accordance with the present invention, it has been foundthat the ionic oxidation proceeds under even milder conditions (circa20° C. and below) when the crude reaction mixture from the autoxidationis employed without purification other than removal of solvent whencompared to oxidizing an isolated purified form of the phthalide. Thisaspect of the present invention will be discussed in detail below.

SUMMARY OF THE INVENTION

This invention comprises a method for converting 3-alkanoyloxyphthalidesselected from the formulae consisting of ##STR1## to the correspondingpolycarboxylic acids having the formulae ##STR2## respectively, whereinR represents lower-alkyl, R₁ is selected from the group consisting ofhydrogen and linear lower-alkyl, A and B taken separately areindependently selected from the group consisting of hydrogen and inertsubstituents, A and B taken together represent an aromatic nucleus fusedto the phenyl ring, and X is selected from the group consisting of--CO--, --SO₂ --, --O--, and a single bond, said method comprisingoxidizing said 3-alkanoyloxyphthalide with aqueous alkaline hypohalite.

This invention also comprises a method for converting aromatic compoundsselected from the formulae consisting of ##STR3## to the corresponding3-alkanoyloxyphthalides defined by (I) and (II) above wherein R₂ and R₃are independently selected from linear lower-alkyl, and the R₁ linearlower-alkyl has one less methylene than the corresponding R₂ or R₃group, and Y is selected from the group consisting of --CO--, --SO₂ --,--O--, a single bond, and --CH₂ --, and A and B have the same meaning asset forth above, said method comprising autoxidizing said aromaticcompound in a solution comprising an anhydride of an aliphaticmonocarboxylic acid having 2 to 8 carbon atoms with oxygen in thepresence of a heavy metal oxidation catalyst and a promoter.

This invention also comprises a method for the conversion of the3-alkanoyloxyphthalides (I) and (II) to the corresponding polycarboxylicacids (III) and (IV) wherein said starting compounds (I) and (II) areprepared by the autoxidation method defined above for the conversion ofthe aromatic compounds (V) and (VI) to (I) and (II) respectively.

This invention also comprises the novel 3-alkanoyloxyphthalides definedby formula (II) above.

The term "lower-alkyl" means alkyl having from 1 to 8 carbon atoms,inclusive, such as methyl, ethyl, propyl, butyl, amyl, hexyl, heptyl,octyl, and isomeric forms thereof.

The term "linear lower-alkyl" means alkyl having the same carbon atomlimitation set forth above but limited to the linear alkyl groupsrecited above.

The term "inert substituent" means any substituent that does not reactwith the carboxylic acid products or phthalides or otherwise interferewith the oxidation methods and is typically inclusive of both lower andlinear lower-alkyl defined above; halo, i.e., chloro, bromo, fluoro, andiodo; alkoxy from 1 to 8 carbon atoms, inclusive, such as methoxy,ethoxy, propoxy, butoxy, pentyloxy, hexyloxy, heptyloxy, octyloxy, andthe like, including isomeric forms thereof; cyano; --SO₃ H; --COOH;--COOR wherein R represents lower-alkyl, aryl, aralkyl, cycloalkyl, andthe like.

The term "aromatic nucleus" means another aromatic hydrocarbon residuefused to the phenyl ring such that the resulting fused ring is typicallyinclusive of naphthalene, phenanthrene, anthracene, fluorene,acenaphthene, and the like.

The term "heavy metal oxidation catalyst" means a heavy metal capable ofexistence in variable valence states and in either elemental, combined,or ionic form which is capable of catalyzing the autoxidation of alower-alkyl group substituted on an aromatic nucleus to a carboxylicacid group, said heavy metals being inclusive of manganese, cobalt,nickel, chromium, vanadium, molybdenum, tungsten, tin, and cerium.

The term "promoter" means an additive which, in combination with theheavy metal oxidation catalyst, is capable of increasing the rate of theautoxidation described above.

The polycarboxylic acid products (III) and (IV) produced in accordancewith the present invention are useful as intermediates in the productionof polyester polymers and alkyds but find particular utility in theproduction of the corresponding anhydrides which in turn serve asintermediates in the preparation of polyimides, polyamides,polyamideimides, curatives for epoxy resins, and the like. Particularlyuseful in this regard are the dianhydrides prepared from thetetracarboxylic acids (IV) produced in accordance with the presentinvention.

The phthalide compounds find particular utility as starting materialsfor the corresponding polycarboxylic acids (III) and (IV) which can beused in the applications noted above.

DETAILED DESCRIPTION OF THE INVENTION

The individual methods in accordance with the present invention will bereferred to hereinafter as Method A and Method B. The combination of thetwo methods, which combination is also in accordance with the presentinvention, can be represented by the following schematic equationsequence starting with a simple ortho-dialkyl-substituted benzene forpurposes of illustration. ##STR4## wherein R₂, R₃, R₁, and R are definedabove. It is to be understood that the ortho-dialkyl-benzene isrepresentative of (V) and (VI) and the phthalide is representative of(I) and (II) while the phthalic acid is representative of (III) and (IV)all as defined above.

Method A

The autoxidation of the aromatic compounds (V) and (VI) to thecorresponding 3-alkanoyloxyphthalides (I) and (II) is carried out insolution using oxygen or an oxygen containing gas such as air in thepresence of, known heavy metal oxidation catalysts and promoters.However, the novelty in the present Method A resides in the choice ofsolvent in which to carry out the solution oxidation.

Surprisingly, the use of the class of acid anhydrides defined above inwhich to carry out the solution oxidation results in a dramatic changein the course of the autoxidation and the resultant products obtained incomparison to prior art methods. Instead of the alkyl groups R₂ and R₃being oxidized directly to carboxylic acids, a 3-alkanoyloxyphthalide isformed. At the same time the anhydride solvent acts as both adehydrating agent and acetylating agent such that the alkanoyl radical(R) appearing on the phthalide ring corresponds to the alkanoyl groupingof the anhydride.

While not wishing the present invention to be bound by any theoreticalconsiderations but only by the claims appended hereinbelow, it isbelieved that approximately 3 molar equivalents of anhydride arerequired per molar equivalent of ortho-dialkyl groups (R₂ and R₃) in (V)and (VI). The pathway from (V), (VI) to (I), (II) can be representedschematically as follows using the same ortho-dialky-substituted benzenemodel compound as above and starting the autoxidation at the R₂ alkylgroup. ##STR5##

It will be obvious to one skilled in the art that, when the linearlower-alkyl group R₂ (or R₃ whichever reacts first) is other thanmethyl, this group is oxidized at the methylene adjacent to the aromaticring, and the lower-alkyl group R₁, in the resulting phthaliderepresents alkyl having one less methylene than R₂.

In this connection, it should be noted that geometric isomer forms ofthe 3-alkanoyloxyphthalides (I) and (II) can be formed in accordancewith the present Method A, depending on which group R₂ or R₃ is oxidizedfirst, and, of course, on the nature of the substituents, if any, on thebenzene ring which would provide the necessary molecular conformationwith the phthalide to result in isomer formation. The autoxidation of(V) and (VI) in accordance with the present invention produces thephthalides (I) and (II) as the major proportion of the reaction product.At the same time, the fully oxidized polycarboxylic acid in the form ofthe anhydride is formed in a minor proportion. Generally speaking, thephthalides are formed in at least a 60 percent molar proportion over thepolycarboxylic acid anhydride. Notwithstanding, the overall autoxidationprocess provides fully oxidized products whether in the form of thephthalides or the fully oxidized carboxylic acid anhydrides with littleor no partially oxidized products. Furthermore, overall conversions andyields are very high with conversions being in excess of 90 percent andproduct yields being well over 90 percent, particularly whenpolycarboxylic acids are the ultimately desired product.

A preferred class of anhydrides for use in the present invention is thatof an aliphatic monocarboxylic acid having 2 to 4 carbon atoms, and,most preferably, acetic anhydride and propionic anhydride.

Generally speaking, the anhydride is employed in quantities sufficientto act as a solvent for the compounds (V) and (VI). The specific amountwhich may be optimum for any given starting compound can be easilydetermined by trial and error. However, as noted above, and, because itdoes play a chemical role in the overall process, it is advantageous toemploy the anhydride in an amount such that at least 3 molar equivalentsper molar equivalent of ortho-dialkyl groups in (V) and (VI) are presentwherein the equivalent weight of the ortho-dialkyl compounds are definedby the molecular weight divided by the number of pairs of ortho-dialkylgroups contained therein.

Preferably, the anhydride is employed in an excess of from about 10 toabout 500 molar percent over that set forth above, and, most preferably,from about 10 to about 60 percent excess.

A preferred solution environment for the autoxidation in accordance withthe present invention comprises a combination of the above definedanhydrides with an aliphatic monocarboxylic acid having 2 to 8 carbonatoms. A preferred combination within this class comprises an anhydridehaving 2 to 4 carbons with the corresponding monocarboxylic acid; mostpreferred are the combinations of acetic acid with acetic anhydride andpropionic acid with propionic anhydride.

The proportions of anhydride and monocarboxylic acid are not critical solong as the minimum requirements for the anhydride set forth above arepresent in any given combination. Advantageously, the acid can beemployed in a weight range of from about 10 to about 500 percent of theweight of the anhydride, preferably about 50 to about 250 percent.

Illustrative of the aliphatic monocarboxylic acid anhydrides are aceticanhydride, propionic anhydride, butyric anhydride, isobutyric anhydride,valeric anhydride, caproic anhydride, heptylic anhydride, caprylicanhydride, and the like. Preferred amongst the anhydrides are aceticanhydride, propionic anhydride, and butyric anhydride.

Illustrative of aliphatic carboxylic acids are acetic, propionic,butyric, isobutyric, valeric, caproic, heptylic, caprylic, and the like.Preferred amongst the acids are acetic, propionic, and butyric.

A particularly preferred combination comprises acetic anhydride andacetic acid.

A preferred aromatic compound to be autoxidized in accordance with thepresent invention has the formula (VI) wherein R₂ and R₃ are methyl inall cases and accordingly, the correspondingly produced (II) has R₁equal to hydrogen.

Illustrative of the aromatic compounds (V) are o-xylene,o-diethylbenzene, o-dipropylbenzene, o-dibutylbenzene,o-dipentylbenzene, o-dihexylbenzene, o-diheptylbenzene,o-dioctylbenzene, 1-methyl-2-ethylbenzene, 4-chloro-o-xylene,4-methoxy-o-xylene, 4-bromo-o-xylene, 3,4-dimethylbenzoic acid;pseudocumene, durene, and the like; 2,3-dimethylnaphthalene,2,3-diethylnaphthalene, 2,3-dimethyl-5-chloronaphthalene,2,3,6,7-tetramethylnaphthalene, 1,2,5,6-tetramethylnaphthalene,1,2-dimethylacenaphthene, 9,10-dimethylphenanthrene,2,3-dimethylfluorene, 2,3,6,7-tetramethylfluorene, and the like.

Illustrative of the aromatic compounds (VI) are2,2',3,3'-tetramethylbiphenyl, 2,2',3,3'-tetraethylbiphenyl,2,2',3,3'-tetrabutylbiphenyl, 3,3',4,4'-tetramethylbiphenyl,3,3',4,4'-tetraethylbiphenyl, 3,4-dimethyl-3',4'-diethylbiphenyl,2,2',3,3'-tetramethyldiphenylmethane,2,2',3,3'-tetraethyldiphenylmethane,2,2',3,3'-tetrabutyldiphenylmethane,2,2',3,3'-tetraoctyldiphenylmethane,2,3-dimethyl-2',3'-diethyldiphenylmethane,3,3',4,4'-tetramethyldiphenylmethane,3,3',4,4'-tetraethyldiphenylmethane, 2,2',3,3'-tetramethyldiphenylether, 2,2',3,3'-tetraethyldiphenyl ether, 3,3',4,4'-tetramethyldiphenylether, 3,3',4,4'-tetraethyldiphenyl ether, 2,2',3,3'-tetramethyldiphenylsulfone, 2,2',3,3'-tetraethyldiphenyl sulfone,3,3',4,4'-tetramethyldiphenyl sulfone, 3,3',4,4'-tetraethyldiphenylsulfone, 3,3',4,4'-tetrabutyldiphenyl sulfone,3,4-dimethyl-3',4'-diethyldiphenyl sulfone,2,2',3,3'-tetramethylbenzophenone, 2,2',3,3'-tetraethylbenzophenone,3,3',4,4'-tetramethylbenzophenone, 3,3',4,4'-tetraethylbenzophenone,3,3'-dimethyl-4,4'-diethylbenzophenone, and the like.

In a preferred embodiment of the Method A in accordance with the presentinvention the aromatic compounds set forth above under (VI) areconverted to the corresponding novel phthalides (II). Particularlypreferred embodiments are those wherein (VI) are the tetralkyldiphenylsulfones and the tetralkylbenzophenones.

Typical, but not limiting of the 3-alkanoyloxyphthalides (I) prepared inaccordance with the present invention are 3-acetoxyphthalide,3-methyl-3-acetoxyphthalide, 3-ethyl-3-acetoxyphthalide,3-propyl-3-acetoxyphthalide, 3-butyl-3-acetoxyphthalide,3-amyl-3-acetoxyphthalide, 3-hexyl-3-acetoxyphthalide,3-heptyl-3-acetoxyphthalide, 3-propanoyloxyphthalide,3-butanoyloxyphthalide, 3-pentanoyloxyphthalide, 3-hexanoyloxyphthalide,3-heptanoyloxyphthalide, 3-octanoyloxyphthalide,3-methyl-3-propanoyloxyphthalide, 3-butyl-3-propanoyloxyphthalide,5-chloro-3-acetoxyphthalide, 5-bromo-3-acetoxyphthalide,5-carboxy-3-acetoxyphthalide, the corresponding bis-3-acetoxyphthalidederived from durene, the corresponding 3-acetoxyphthalide derived from2,3-dimethylnaphthalene, the corresponding bis-3-acetoxyphthalidederived from 2,3,6,7-tetramethylnaphthalene, the corresponding3-acetoxyphthalide derived from 1,2-acenaphthene, and the like.

Typical but not limiting of the 3-alkanoyloxyphthalides (II) are4,4'-bis(3-acetoxyphthalide), 4,7'-bis(3-acetoxyphthalide),7,7'-bis(3-acetoxyphthalide), 4,4'-bis(3-propanoyloxyphthalide),4,7'-bis(3-propanoyloxyphthalide), 7,7'-bis(3-propanoyloxyphthalide),5,5'-bis(3-acetoxyphthalide), 5,6'-bis(3-acetoxyphthalide),6,6'-bis(3-acetoxyphthalide), 4,4'-methylenebis(3-acetoxyphthalide),4,7'-methylenebis(3-acetoxyphthalide),7,7'-methylenebis(3-acetoxyphthalide),7,7'-methylenebis(3-propanoyloxyphthalide),5,5'-methylenebis(3-acetoxyphthalide),5,6'-methylenebis(3-acetoxyphthalide),6,6'-methylenebis(3-acetoxyphthalide), 4,4'-oxybis(3-acetoxyphthalide),4,7'-oxybis(3-acetoxyphthalide), 7,7'-oxybis(3-acetoxyphthalide),5,5'-oxybis(3-acetoxyphthalide), 5,6'-oxybis(3-acetoxyphthalide),6,6'-oxybis(3-acetoxyphthalide), 4,4'-sulfonylbis(3-acetoxyphathalide),4,7'-sulfonylbis(3-acetoxyphthalide),7,7'-sulfonylbis(3-acetoxyphthalide),4,4'-sulfonylbis(3-propanoyloxyphthalide),4,7'-sulfonylbis(3-butanoyloxyphthalide),4,7'-sulfonylbis(3-pentanoyloxyphthalide),7,7'-sulfonylbis(3-propanoyloxyphthalide),7,7'-sulfonylbis(3-butanoyloxyphthalide),7,7'-sulfonylbis(3-pentanoyloxyphthalide), 7,7'-sulfonylbis(3-hexanoyl-oxyphthalide),5,5'-sulfonylbis(3-acetoxyphthalide),5,5'-sulfonylbis(3-propanoyloxyphthalide),5,5'-sulfonylbis(3-butanoyloxyphthalide),5,5'-sulfonylbis(3-pentanoyloxyphthalide),5,5'-sulfonylbis(3-hexanoyloxyphthalide),5,5'-sulfonylbis(3-heptanoyloxyphthalide),5,6'-sulfonylbis(3-acetoxyphthalide),5,6'-sulfonylbis(3-propanoyloxyphthalide),5,6'-sulfonylbis(3-butanoyloxyphthalide),6,6'-sulfonylbis(3-acetoxyphthalide),6,6'-sulfonylbis(3-propanoyloxyphthalide),4,4'-carbonylbis(3-acetoxyphthalide),4,4'-carbonylbis(3-propanoyloxyphthalide),4,4'-carbonylbis(3-butanoyloxyphthalide),4,4'-carbonylbis(3-pentanoyloxyphthalide),4,7'-carbonylbis(3-acetoxyphthalide),4,7'-carbonylbis(3-propanoyloxyphthalide),4,7'-carbonylbis(3-butanoyloxyphthalide),7,7'-carbonylbis(3-acetoxyphthalide),7,7'-carbonylbis(3-propanoyloxyphthalide),5,5'-carbonylbis(3-acetoxyphthalide),5,5'-carbonylbis(3-propanoyloxyphthalide),5,5'-carbonylbis(3-butanoyloxyphthalide),5,5'-carbonylbis(3-pentanoyloxyphthalide),5,6'-carbonylbis(3-acetoxyphthalide),5,6'-carbonylbis(3-butanoyloxyphthalide),5,6'-carbonylbis(3-pentanoyloxyphthalide),6,6'-carbonylbis(3-acetoxyphthalide),6,6'-carbonylbis(3-propanoyloxyphthalide),6,6'-carbonylbis(3-butanoyloxyphthalide),6,6'-carbonylbis(3-pentanoyloxyphthalide), and the like.

Preferred amongst the 3-alkanoyloxyphthalides set forth above asintermediates in the further oxidation to the polycarboxylic acids arethose novel classes of diphthalides falling within formula (II).Particularly preferred amongst those set forth above are thesulfonylbis(phthalides) and carbonylbis(phthalides).

The autoxidation process is carried out in the presence of any type ofheavy metal oxidation catalyst known in the art and defined above.Although the heavy metal can be used in its elemental finely dividedform, or other combined forms, it has been found advantageous to employthe metals in the forms in which the metal ion itself is provided.

Typical of the heavy metal catalysts which can be used in accordancewith the present invention are manganese acetate, cobalt acetate, nickelacetate, chromium acetate, vanadium acetate, molybdenum acetate, tinacetate, ammonium molybdate, cobalt hydroxy quinolate.

Preferred are the heavy metal acetates and particularly preferred iscobaltous acetate.

The amount of catalyst which is most efficacious and economical in anygiven reaction can be easily determined by one skilled in the art bytrial and error testing. Generally speaking, the catalyst is employed inan amount of from about 1 to about 15 mole percent based on the moles ofstarting aromatic compound employed.

Preferably, the catalyst is employed in an amount from about 4 to about10 mole percent.

The promoter as defined above can be any additive which is found toincrease the overall autoxidation rate or otherwise assist in theprocess.

Typical, but not limiting, of promoters are methyl ethyl ketone, ozone,zirconyl acetate, sodium acetate, potassium acetate, barium acetate,zinc acetate, potassium sulfate, titanium dioxide, sources of brominesuch as hydrogen bromide, ammonium bromide, potassium bromate,tetrabromoethane, benzyl bromide, potassium bromide, sodium bromide, andthe like. Also, the bromine can be provided in the same compound as thecatalyst itself such as manganese bromide and the like.

Preferred as promoters are the compounds providing a source of bromine,particularly ionic bromine, such as potassium bromide, sodium bromide,and the like.

The amount of promoter to be employed can vary within wide limits andthe amount of any given promoter providing the most efficacious resultscan be easily determined by trial and error.

Generally speaking, the promoter is employed in an amount of from about1 to about 15 mole percent based on the moles of starting aromaticcompound employed.

In most cases it is advantageous to employ the catalyst and promoter inequimolar proportions.

The autoxidation reaction is carried out readily using typical reactionmethods and apparatus described in the prior art.

Any type of reaction vessel can be employed ranging from laboratoryglassware which is open to atmospheric pressure to sealed autoclavescapable of withstanding high pressures. The particular apparatus to beused depends largely on whether the oxygen containing gas will be underpressure. In this connection, the oxygen can be in the form of the puregas, or admixed with other inert gases including air itself, orcompressed air, and the like.

Preferably, the oxygen is in the form of the pure gas and is used at apressure ranging from atmospheric to just high enough above atmosphericto maintain a positive pressure of oxygen above the reaction mixture.The reaction mixture is preferably agitated by agitation means such asmechanical stirring.

The autoxidation is advantageously carried out within a temperaturerange of about 50° C. to about 250° C., depending largely on thecatalyst employed, the anhydride solvent employed or the mixture thereofwith acid and the rate of autoxidation desired. Preferably the oxidationis carried out at a temperature from about 100° C. to about 150° C.

The time required to effect the optimum conversion of aromatichydrocarbon to phthalide will vary with the substrate employed and theanhydride/acid mixture etc. Again, this variable factor is not limitingin respect of the present invention but is easily determined by oneskilled in the art by trial and error methods.

The progress of the autoxidation can be studied using any convenientanalytical technique. However, a particularly simple and accurate methodcomprises removing an aliquot sample periodically from the reactionmixture and determining the proton nuclear magnetic resonance spectrumof the sample. The change and/or disappearance of the aliphatic protonsadjacent to the aromatic ring and appearance of protons characteristicof the phthalide provides a rapid measure of the total mole percentconversion.

Another convenient means for analyzing the progress of the reaction,and, more particularly, the actual weight percent concentration ofproduct versus starting material is the analysis of an aliquot sample byhigh pressure liquid chromatography (HPLC). (see Modern Practice ofLiquid Chromatography edited by J. J. Kirkland, 1971, WileyIntersciences Div. of John Wiley & sons, Inc., New York, N.Y.).

Other methods of monitoring the reaction are also useful such as thinlayer chromatography (TLC) and gas/liquid chromatography (GPC).

Moreover, such analytical methods provide a simple means for determiningwhen the autoxidation may be stopped conveniently as, for example,evidenced by the complete disappearance of the protons characteristic ofthe aliphatic ones adjacent to the aromatic ring.

The 3-alkanoyloxyphthalides produced in accordance with the aboveprocedures can be easily isolated from the reaction mixtures andobtained in pure form by known methods. At the same time, if anypolycarboxylic acid anhydrides are formed, they too can be isolated byknown methods such as conversion to the sodium salts of the acid andextraction by water from the mixture and then converted to the freeacid, or alternatively, if desired, back to the anhydride by knownmethods.

Illustratively, the solvent anhydride or mixture thereof with acid canbe removed by distillation procedures and the phthalides and anhydridesisolated in crude form from which the pure materials can be removed fromthe catalyst and promoter by extraction by organic solvent followed byremoval of the solvent by known methods.

Alternatively, the reaction mixture, including the solvent, can betreated with a non-solvent for the phthalide and anhydride causing themto precipitate as a solid which is then collected by known filtrationtechniques. The polycarboxylic acid anhydride can be separated by theextraction procedure noted above.

In a preferred embodiment to be discussed in detail below in connectionwith the combination of Methods A and B, the solvent(s) are simplyremoved by known distillation methods to provide a crude reactionproduct which contains the catalyst and promoter as impurity along withthe phthalide and minor amount of polycarboxylic acid anhydride whichcrude reaction product is advantageously employed, without furthertreatment, in Method B for conversion of both products to thecorresponding polycarboxylic acid.

Method B

The ionic oxidation of compounds (I) and (II) is carried out in aqueousalkaline hypohalite solution to form the corresponding carboxylic acids(III) and (IV). A particular advantage of the method is that theoxidation proceeds at low temperatures, even as low as room temperatureand below.

Using the model 3-alkanoyloxyphthalide that was employed illustrativelyabove, the ionic oxidation in accordance with the present invention canbe represented schematically as follows. ##STR6## wherein R and R₁ aredefined above and MOX represents a hypohalite reactant wherein Xrepresents halogen such as chlorine, bromine, and iodine, and Mrepresents an alkali or alkaline earth metal such as sodium, potassium,lithium, and calcium.

In a preferred embodiment of the Method B in accordance with the presentinvention the preferred class of novel diphthalides having the formula(II) above are oxidized to the corresponding tetracarboxylic acids (IV).

Particularly preferred embodiments are those wherein the startingdiphthalides are the sulfonylbis(phthalides) and carbonylbis(phthalides)including isomer mixtures of the respective groups.

It will be obvious to one skilled in the art that the aromaticcarboxylic acids produced in accordance with this method include suchtypical ones as phthalic acid, 4-halogenophthalic acid (such aschlorine, bromine, iodine, fluorine), 4-methoxyphthalic acid,trimellitic acid, pyromellitic acid, naphthalene-2,3-dicarboxylic acid,naphthalene-2,3,6,7-tetracarboxylic acid, acenaphthene-1,2-dicarboxylicacid, 2,2',3,3'-biphenyltetracarboxylic acid,3,3',4,4'-biphenyltetracarboxylic acid, 3,3'-methylenebis(phthalicacid), 4,4'-methylenebis(phthalic acid),3,3',4,4'-diphenylethertetracarboxylic acid,2,2',3,3'-diphenylethertetracarboxylic acid,2,2',3,3'-diphenylsulfonetetracarboxylic acid,3,3',4,4'-diphenylsulfonetetracarboxylic acid,2,2',3,3'-benzophenonetetracarboxylic acid,3,3',4,4'-benzophenonetetracarboxylic acid, and the like.

In the practice of this Method B, the 3-alkanoyloxyphthalide is made todisperse or preferably dissolve in an aqueous alkaline hypohalitesolution and the resulting product is stirred. The reaction solution ismaintained on the alkaline side throughout the course of the reaction inthe manner described below.

The mode of operation in respect of whether the aqueous alkalinehypohalite is prepared in a separate step and added to the phthalide,or, alternatively, prepared in situ by the addition of the appropriateelemental halogen directly to the reaction solution of aqueous alkalinephthalide is purely optional. As far as ease of manipulation it ispreferable to prepare the hypohalite in situ.

The hypohalite is advantageously employed within a range of about 0.75to about 2.0 moles per equivalent of phthalide, and preferably within arange of from about 1.0 to about 1.5 moles per equivalent of phthalidewherein the equivalent weight of the phthalide is defined by itsmolecular weight divided by the number of phthalide rings in themolecule.

The hypohalite employed can be any one of the known materials which havebeen used in the prior art for carrying out ionic oxidations and isinclusive of sodium hypochlorite, sodium hypobromite, sodium hypoiodite,potassium hypochlorite, potassium hypobromite, potassium hypoiodite,calcium hypochlorite, calcium hypobromite, and calcium hypoiodite.Preferred amongst the illustrative examples above are sodiumhypochlorite, potassium hypochlorite, and calcium hypochlorite.

As noted previously, it is preferred to prepare the hypohalite in situsimply by adding the halogen reagent, for example chlorine, into asolution of the phthalide in aqueous base until an amount required toprepare a predetermined proportion of hypohalite has been reached.

The aqueous base can be any of the strong inorganic bases known to thoseskilled in the art and typical of such reagents are sodium hydroxide,potassium hydroxide, lithium hydroxide, and the like. Conveniently, thebase reagent and the hypohalite have the same cation which is the casewhen the hypohalite is prepared in situ.

The specific amounts of basic reagent to be employed to keep thesolution alkaline will vary somewhat depending on such factors as thenumber of phthalide rings to be oxidized, whether the hypohalite is tobe generated in situ, and the like. However, it is a simple matter todetermine the requisite amount of base to maintain the alkalinity of thereaction solution and adjust it accordingly.

Simply as a guide but not a limiting amount, the base reagent can beemployed at a level of at least about 4 equivalents per equivalent ofphthalide. The presence of excess is particularly advantageous when thehypohalite is prepared in situ because of the consumption of base in thepreparation of the hypohalite. For example, two equivalents of sodiumhydroxide are required for every equivalent of sodium hypochloriteprepared from reaction with chlorine.

The oxidation is carried out in any suitable vessel capable of beingstirred and heated. The temperature at which the ionic reaction isperformed is surprisingly mild and will vary somewhat depending on thestarting phthalide, particularly its purity and the presence ofingredients which appear to have a catalytic effect on the oxidation andwhich ingredients and the circumstances of their presence will bediscussed below. Advantageously, the oxidation is carried out at atemperature of from about 0° C. to about 100° C., and preferably fromabout 20° C. to about 75° C.

The progress of the oxidation can be followed using the same analyticalprocedures described above for Method A. TLC analysis serves as a simpleand rapid means for determining the progress and completion of thereaction. Advantageously, the reaction is completed within a period offrom about 1 hour to about 8 hours, preferably from about 1 hour toabout 4 hours.

As noted above the polycarboxylic acid is obtained directly, and, mosteasily, simply by neutralizing the basic reaction solution with any acidreagent capable of neutralizing the base. The free carboxylic acids,generally speaking, precipitate directly from the aqueous solution ascrystalline products which are easily collected by standard filtrationtechniques.

It should be noted that both an isomer mixture and a single form of thephthalide starting material produce the same polycarboxylic acidproduct.

Further, it should be noted that, if the mixture to be oxidized consistsof phthalide(s) and its(their) corresponding polycarboxylic acidanhydride(s), the sole product(s) is(are) the correspondingpolycarboxylic acid(s).

Combination of Method A and Method B

Although the starting phthalides (I) and (II) used in the oxidationMethod B can be obtained via alternate synthetic routes, the Method Adescribed above provides the materials in unexpectedly high yield andpurity. Furthermore, the side-product obtained, namely the acidanhydride of the polycarboxylic acid, serves as a perfect startingmaterial in Method B because it becomes part of the polycarboxylic acidyield along with the same polycarboxylic acid formed from the phthalide.

In an optional, but much preferred, embodiment of the overall oxidationof aromatic hydrocarbons (V) and (VI) to the correspondingpolycarboxylic acid (III) and (IV) it has been discovered, quitesurprisingly, that Method A and B can be combined in such a way that theMethod B oxidation can be carried out at room temperature and stillprovide the acids in high yields.

Illustratively, after the autoxidation of the aromatic hydrocarbon hasbeen completed in accordance with Method A set forth above, the reactionsolution is treated to remove the solvent, for example, by distillation.

The crude reaction mixture obtained can be in the form of an oil orsemi-solid, and, in some cases a crystalline mass. It still contains theheavy metal oxidation catalyst and promoter. This crude reaction mixturecontaining the phthalide, the catalyst and promoter, and, anypolycarboxylic acid anhydride which may have formed, is used directly inthe Method B oxidation described above.

Although the ionic oxidation can be carried out at any temperaturewithin the range set forth above (i.e. 0° to 100° C.), it has been foundthat the oxidation proceeds rapidly at ambient room temperature (about20° C.).

The catalyst and promoter combination which seems to provide thepreferred enhancement of the Method B is the combination of at least oneheavy metal acetate and at least one bromide ion promoter such assodium, potassium, and lithium bromide. Most preferred amongst thesegroups is the combination of cobalt acetate with sodium bromide.

The polycarboxylic acids obtained in accordance with the presentinvention have the various utilities noted above. In particular, theyare readily converted to the acid anhydride using any of the standardmethods employed for such a conversion. Typically, the acids can beheated under reflux in organic solvents, as for example,1,2,4-trichlorobenzene or 1,2-dichlorobenzene and the water formed bythe anhydride formation removed using a Dean-Stark trap.

The anhydrides are particularly useful in the preparation of polymerssuch as polyamides, polyamideimides, polyimides and the like.

The following examples describe the manner and process of making andusing the invention and set forth the best mode contemplated by theinventor for carrying out the invention but are not to be construed aslimiting.

EXAMPLE 1

A three-necked flask was provided with a condenser, a thermometer, and agas inlet tube and was charged with 24.0 g. (0.1008 mole) of3,3',4,4'-tetramethylbenzophenone which was 97 to 98 percent pure, 1.6g. (0.0064 mole) of cobaltous acetate tetrahydrate, 0.8 g. (0.0078 mole)of sodium bromide, 80 g. (0.784 mole) of acetic anhydride, and 120 ml.of glacial acetic acid.

The reaction mixture, which was a suspension at room temperature, wasstirred and heating with an oil bath was initiated. The mixture soonbecame a homogeneous solution and starting at 60° C. a stream of oxygenat a rate of about 150 to 200 ml. per minute was passed into thesolution. The oil bath was maintained at a temperature of 150° C. to155° C. which resulted in a solution temperature of 120° C. to 125° C.

After 7 hours the solution temperature dropped to 115° C. Thin layerchromatography (TLC) of an aliquot sample which was spotted on a silicagel plate (5 cm×10 cm KF5 plate supplied by Whatman Filter Co., Clifton,N.J.) and developed with a 6/4 parts by weight solution of ethyl acetateand cyclohexane showed the presence of 3 components (by ultavioletlight) corresponding to the three geometric isomers of 5,5'-, 5,6'-, and6,6'-carbonylbis(3-acetoxyphthalide). Another aliquot, taken at sametime, spotted on a TLC plate and developed with a 7/3 mixture of ethylacetate and acetic acid showed two components corresponding to3,3',4,4'-benzophenonetetracarboxylic acid dianhydride and thehydrolyzed 3,3',4,4'-benzophenonetetracarboxylic acid.

The autoxidation solution was poured into 400 ml. of ice cold waterduring stirring. A white solid precipitate was formed which was isolatedby filtration, washed with ice water and pressed and dried to provide awhite solid autoxidate product; wt.=38.0 g. Proton NMR showed a sharpsinglet at δ7.56 (benzylic proton), and two closely situated singlets atδ62.20 and 2.16 (acetate protons). Based on the ratio of aromaticprotons to benzylic protons the isolated autoxidate was a 70/30 molepercent mixture of the 3 phthalide isomers noted above and3,3',4,4'-benzophenonetetracarboxylic acid dianhydride respectively.

The 38.0 g. of solid autoxidate was transferred to a three-necked flaskequipped with a high speed mechanical stirrer, a thermometer and acondenser. To the flask was added 300 ml. of aqueous alkaline sodiumhypochlorite in one portion. The hypohalite had been previously preparedby passing 12.2 g. (0.172 mole) chlorine into a sodium hydroxidesolution [prepared from 42.0 g. (1.05 mole) of sodium hydroxidedissolved in 300 ml. water]. The reaction solution was stirred atambient temperature (circa 20° C.) for 2 hours, followed by stirring at50°-55° C. for 1 hour.

The cooled solution was acidified with 75 ml. concentrated hydrochloricacid, cooled in ice, and seeded with a sample of3,3',4,4'-benzophenonetetracarboxylic acid. A white solid precipitate ofthe 3,3',4,4'-benzophenonetetracarboxylic acid formed. The crystallineproduct was collected by filtration, washed with 30 to 40 ml. ofice-cold water, and dried in an oven at 100° C.; dry product wt.=30.0 g.(83.1% yield based on the tetramethylbenzophenone); m.p. 218°-220° C.(dec.).

The autoxidation reaction described above was repeated using the sameingredients, proportions, and conditions, up to the end of the 7 hourreaction period. At this point the reaction solution was heated at 50°C. under aspirator pressure (about 10 mm of mercury pressure) and thesolvent stripped off.

A residue of 47.0 g. of crude reaction mixture in the form of tarryviscous material remained. This residue was subjected to the same ionicoxidizing conditions set forth above using the same concentration ofalkaline sodium hypochlorite.

The reaction solution was stirred briskly to dissolve the tarry residueat 15° C. for one hour followed by room temperature (circa 20° C.) for 3hours.

A black precipitate of cobalt oxide was removed by filtering with CeliteFilter Aid (supplied by Johns-Manville Products Corp., Lompoc, Cal.).The filtrate was acidified with 75 ml. conc. hydrochloric acid andseeded with 3,3',4,4'-benzophenonetetracarboxylic acid. The precipitated3,3',4,4'-benzophenonetetracarboxylic acid product was collected anddried at 100° C.; wt.=32.0 g. (88.7% yield based on startingtetramethylbenzophenone); m.p. 218°-220° (dec.).

EXAMPLE 2

Using the procedure and apparatus set forth above in Example 1, theautoxidation of 3,3',4,4'-tetramethylbenzophenone was repeated exceptthat the proportions of all the reactants were halved. After the 7 hourreaction period had elapsed the solvent was stripped off in vacuoaccording to the method described above to provide 23.9 g. of autoxidateresidue.

The residue was scraped and transferred to a reaction flask equipped asdescribed above in Example 1 for the ionic oxidation. Sodium hydroxidesolution prepared from 23 g. of sodium hydroxide dissolved in 175 ml. ofwater was added to the flask and stirred at 15° C. for 1 hour using acooling bath. Chlorine (6 g.) was passed into the stirred solution andafter the addition was completed the cooling bath was removed.

The solution was stirred for 3 hours at room temperature (circa 20° C.)during which period the temperature reached 32° C. after one hour andslowly receded to 27° C. A TLC analysis of an aliquot as describedabovein Example 1 on silica gel and development in 7/3 ethylacetate/acetic acid showed that the reaction was complete.

The solution was filtered to remove the black cobalt oxide and thefiltrate acidified with 36 ml. of conc. hydrochloric acid. After seedingwith pure 3,3',4,4'-benzophenonetetracarboxylic acid and standing atroom temperature (20° C.) the product of3,3',4,4'-benzophenonetetracarboxylic acid precipitated and wascollected and dried to constant weight=16.6 g. (92.2% yield based onstarting tetramethylbenzophenone).

EXAMPLE 3

Using the apparatus and procedure described in Example 1, 12.0 g.(0.0504 mole) of 3,3',4,4'-tetramethylbenzophenone of 99% purity wasautoxidized using 1.0 g. (0.004 mole) of cobaltous acetate tetrahydrate,0.5 g. (0.005 mole) of sodium bromide and 100 ml. of acetic anhydride.The oxygen was passed into the solution at a rate of about 100ml./minute during stirring and while heating the solution in an oilbath.

The reaction temperature after 20 minutes was about 130° C. and, over aperiod of about 4 hours with a reaction temperature of about 124° C.,the oxidation was shown to be complete by the TLC analysis method setforth above using the silica gel plate and development solution of 6/4ethyl acetate/cyclohexane. The products shown to be present were the5,5'-, 5,6'-, and 6,6'-carbonylbis(3-acetoxyphthalide) isomer mixtureand 3,3',4,4'-benzo-phenonetetracarboxylic acid dianhydride.

The resulting blue solution was diluted with water, extracted with ethylacetate and the organic layer separated and dried by storage overmagnesium sulfate. Removal of solvent in vacuo on a rotary evaporatorunder aspirator pressure followed by vacuum pump pressure yielded 20.0g. of tarry residue. Proton NMR showed the residue to be a 75/25 molepercent mixture of the 5,5'-, 5,6'-, and6,6'-carbonylbis(3-acetoxyphthalide) isomer mixture and3,3',4,4'-benzophenonetetracarboxylic acid dianhydride. The overallyield of oxidized products was 100%.

EXAMPLE 4

Using the apparatus and procedure described in Example 1, 13.7 g. (0.05mole) of 3,3',4,4'-tetramethyldiphenylsulfone of 88.9% purity wasautoxidized using 1.0 g. (0.004 mole) of cobaltous acetate tetrahydrate,0.5 g. (0.005 mole) of sodium bromide and 100 ml. of acetic anhydride.The oxygen was passed into the solution at a rate of about 100ml./minute during stirring and while maintaining the solution at atemperature of 120° C. to 127° C.

After a 3 hour reaction period, the solution was cooled and the solventremoved in vacuo to yield a semi-solid residue.

The semi-solid autoxidate residue was transferred to a reaction flaskequipped as described above in Example 1 for the ionic oxidation. A 175ml. portion of aqueous alkaline sodium hypochlorite was added to theresidue. The hypohalite had been previously prepared by passing 7.1 g.of chlorine into a solution of 22 g. of sodium hydroxide dissolved in175 ml. of water.

The reaction flask was cooled to 10° C. to 15° C. by means of a coolingbath and stirred for about 1 hour to dissolve the autoxidate residue.The cooling bath was removed and the solution stirred at roomtemperature (circa 20° C.) for 2 hours during which time the solutiontemperature reached 45° C. and then receded to 32° C. TLC analysis of analiquot sample indicated that the oxidation was completed after 1 hour.

A black precipitate of cobalt oxide was removed by filtration and thefiltrate was acidified with 37 ml. of conc. hydrochloric acid. Uponcooling, a white crystalline precipitate of the3,3',4,4'-diphenylsulfonetetracarboxylic acid was formed. It wasisolated by filtration, washed with a 10 percent sodium chloridesolution, then dried at 100° C.; wt.=14.0 g. (81% yield based on theadjusted weight of pure starting tetramethyldiphenylsulfone); m.p. 240°C.-244° C. (dec.); ¹³ C NMR confirmed the structure as the3,3',4,4'-diphenylsulfonetetracarboxylic acid.

EXAMPLE 5

Using the apparatus and procedure described in Example 1, 10.6 g. (0.10mole) of o-xylene was autoxidized using 1.0 g. (0.004 mole) of cobaltousacetate, 0.5 g. (0.005 mole) of sodium bromide and 100 ml. of aceticanhydride. The oxygen was passed into the solution at a rate of about100 ml./minute during stirring and while heating the solution at atemperature of 110° C. to 122° C.

After 3 hours gas liquid chromatography (GLC) analysis of an aliquotsample showed the presence of two components equivalent to a 60 weightpercent proportion of 3-acetoxyphthalide and 40 weight percentproportion of phthalic anhydride.

Continuation of the oxidation for a further period of about 1.5 hoursresulted in a change of the weight concentrations of the above twoproducts to 56 percent and 44 percent respectively.

The reaction solution was diluted with water, extracted with ethylacetate, the organic layer washed with water, then dried by storage overanhydrous magnesium sulfate. Evaporation of the solvent in a rotaryevaporator under aspirator pressure and a warm water bath provided acrystalline solid residue; wt.=15.0 g. (GLC analysis showed a 55/45weight percent proportion of 3-acetoxyphthalide to phthalic anhydride;proton NMR and GLC comparisons with authentic samples confirmed theidentities of these two products); yield of 3-acetoxyphthalide=42.9%;yield of phthalic anhydride=45.6%.

The mixture of 3-acetoxyphthalide and phthalic anhydride is oxidized bythe ionic oxidation procedure set forth in Example 1 using aqueousalkaline sodium hypochlorite solution. Acidification with concentratedhydrochloric acid and cooling of the resulting reaction mixture providesthe precipitated phthalic acid.

I claim:
 1. A method for converting a 3-alkanolyloxyphthalide selected from the formulae consisting of ##STR7## to the corresponding polycarboxylic acids having the formulae ##STR8## wherein R represents lower-alkyl, R₁ is selected from the group consisting of hydrogen and linear lower-alkyl, A and B taken separately are independently selected from the group consisting of hydrogen and any substituent that does not react with the carboxylic acid products or phthalides or otherwise interfere with the oxidation, A and B taken together represent an aromatic nucleus fused to the phenyl ring, and X is selected from the group consisting of --CO--, --SO₂ --, --O--, and a single bond, said method comprising oxidizing said 3-alkanoyloxyphthalide with aqueous alkaline hypohalite.
 2. A method according to claim 1 wherein said hypohalite is sodium hypochlorite.
 3. A method according to claim 1 wherein an alkanoyloxyphthalide having the formula (II) wherein R₁ is hydrogen is converted to the corresponding tetracarboxylic acid (IV).
 4. A method according to claim 3 wherein X is --CO--.
 5. A method according to claim 3 wherein X is --SO₂ --.
 6. A method according to claim 3 wherein R is methyl and said oxidation is carried out with aqueous alkaline sodium hypochlorite at a temperature of from about 20° C. to about 75° C.
 7. A method according to claim 6 wherein a mixture of isomers comprising 5,5'-, 5,6'-, and 6,6'-carbonylbis(3-acetoxyphthalide) is converted to 3,3',4,4'-benzophenonetetracarboxylic acid.
 8. A method according to claim 6 wherein a mixture of isomers comprising 5,5'-, 5,6'-, and 6,6'-sulfonylbis (3-acetoxyphthalide) is converted to 3,3',4,4'-diphenylsulfonetetracarboxylic acid.
 9. A method in accordance with claim 1 wherein said 3-alkanoyloxyphthalide employed as starting material is prepared by autoxidizing with oxygen in a solution comprising an anhydride of an aliphatic monocarboxylic acid having 2 to 8 carbon atoms in the presence of a heavy metal oxidation catalyst and a promoter an aromatic compound selected from the formulae consisting of ##STR9## wherein R₂ and R₃ are independently selected from linear lower-alkyl, A and B when taken separately are independently selected from the group consisting of hydrogen and any substituent that does not react with the carboxylic acid products or phthalides or otherwise interfere with the oxidation, A and B when taken together represent an aromatic nucleus fused to the phenyl ring, and Y is selected from the group consisting of --CO--, --SO₂ --, --O--, a single bond, and --CH₂ --.
 10. A method according to claim 9 wherein 3,3',4,4'-tetramethylbenzophenone is converted to 3,3',4,4'-benzophenonetetracarboxylic acid by(a) autoxidizing said 3,3',4,4'-tetramethylbenzophenone with oxygen at a temperature of from about 100° C. to about 125° C. in a solution comprising a mixture of acetic acid and acetic anhydride in the presence of cobaltous acetate catalyst and sodium bromide promoter to form a crude reaction mixture comprising a major proportion of a mixture of 5,5'-, 5,6'-, and 6,6'-carbonylbis(3-acetoxyphthalide) and a minor proportion of 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride; (b) removing said acetic acid and acetic anhydride from said crude reaction mixture; and (c) oxidizing said crude reaction mixture with aqueous alkaline sodium hypochlorite to form said 3,3',4,4'-benzophenonetetracarboxylic acid. 